628 Various Slow-speed Engines. where a vacuum is formed and maintained by the action of the air pump h. Fig. 608 shews that water under low pressure (as in a condenser) will boil and form vapour at a low temperature ; and the air pump has to remove this vapour as far as possible, as well as the condensation water. Even then there is always a back pressure of 3 or 4 Ibs. per sq. in. When the piston descends, valves b and d are open and c closed, there then being boiler steam at top and a vacuum below ; during the upstroke, b and d are closed and c is open, which places the piston in equilibrium," when the pump rods raise it by their weight. The parallel motion (Watt's) is explained at p. 499 ; but in A, Fig. 626, one radius link is formed by the portion t k of the beam, and a parallelogram then connected to the middle link kb^ so that the valve and piston rods move on parallel lines. A rotative beam engine is shewn at B. It differs from A in having the crank and connecting rod instead of pump rod, and four drop valves instead of three, the reason being that each end of the cylinder must now be connectable with boiler or condenser at will, and must therefore have a steam and exhaust valve. The method of distribution is given in Fig. 629, where the left pipe admits live steam to either end of cylinder, and the right pipe similarly removes the .exhaust steam, whenever the proper valves are lifted. A direct-acting pumping engine like that at c may have a beam solely for actuating the valves and air pump, though it also serves to guide the piston-rod. The straight line motion is Scott-Russell's (see p. 486), sometimes 'called ' grasshopper' gear. A beam blowing engine is shewn at D, a being the steam cylinder and b the blowing cylinder, the latter having inlet valves dd> and outlet valves <?<?, for both ends, so that the issuing air may pass continuously to the blast furnace or other place of use. The fly-wheel is introduced to steady the motion, E is a compound beaju engine. The high-pressure cylinder a is placed nearest the beam trunnion, and the low-pressure cylinder further outward. The valves are not shewn, but are so arranged that, when the steam has done its work in the H.P. cylinder, it is allowed to expand into the L.P. cylinder before passing to, the condenser.